Objective: The hypothesis that human cancer may be associated with an infectious agent transmitted from person to person following repeated exposure to patients with cancer is the basis for this study. Approach: A study of yearly cohorts of physicians and lawyers who have been registered in Connecticut from 1900 to 1975 will be undertaken in order to compare mortality patterns and causes of death among individuals from these cohorts presumably sharing similar socio-economic background but having different professional activities. The life experience of each yearly cohort of physicians and lawyers from the time of first registration with the Division of Medical Licensure or the Connecticut Bar will be recorded and compared. Physicians will be studied according to "person-years of exposure" to various specialties practiced during their careers. Life expectancy and cause of death of lawyers will be compared with that of physicians belonging to the same registration year cohort. Further analyses will be made for physicians according to their number of years experience in different specialties.